The invention relates to a switch for optical signals and more particularly to a switch for packet signals.
Telecommunications are expanding considerably. More and more users (individuals and businesses) are transmitting an increasing number of messages in telecommunication networks. Also, the messages include an ever-increasing quantity of information, for example when sending pictures. To respond to this growing demand for information bit rate, telecommunications network operators are using optical signal transmission, which modulates optical signals, generally produced by lasers, in accordance with the information to be transmitted, after which the modulated signals propagate in a network of conductors or optical fibers.
Transmitting signals optically has several advantages. In particular, the attenuation of the signal during transmission is less than in the case of electrical signals and the bandwidth of optical fibers is greater. It is therefore possible to transmit several carriers with different wavelengths simultaneously in the same fiber. This technique, known as wavelength division multiplexing, achieves information bit rates of the order of 1 terabit/s.
In parallel with wavelength division multiplexing, time division multiplexing enables simultaneous transmission of several calls on the same carrier. In packet mode, each carrier transmits packets relating to different messages whose information has been divided up into packets, each packet being launched into the network with a header indicating its destination. When the packet passes through a switching device, the device dedicates resources to routing the packet during the time period needed to switch the packet to a requested output. Those resources are then freed again for switching another packet. Because the packets are of limited duration, of the order of 1 microsecond, many calls can be transmitted in a short time period. This routing policy is currently used on the largest of all networks: the Internet.
Implementing switches using optical technology has been envisaged. Two types of switch can be distinguished:
cross-connect switches that set up semi-permanent connections between trunks routing a large number of multiplexed messages or calls, and
switches capable of routing calls or messages individually, i.e. that can be reconfigured for each new call or each new message.
The document JINNO M ET AL.: xe2x80x9cULTRA-WIDE-BAND WDM NETWORKS AND SUPPORTING TECHNOLOGIESxe2x80x9d CORE NETWORKS AND NETWORK MANAGEMENT, AMSTERDAM: IOS PRESS, NL, 1999, pages 90-97, XP000829416 ISBN/90-5199-497-4 describes a cross-connect switch that receives and switches, without demultiplexing them, optical signals consisting of 16 carriers having different wavelengths. This switch cannot route individual calls or messages. Also, it is designed for very low switching speeds.
The invention relates to switches whose function is to route calls or messages individually. Also, if the signals are transmitted in packet mode, the switch must route all the packets present at the respective inputs to designated outputs and then change configuration to route subsequent packets.
FIG. 1 is a block diagram of a prior art switch 10 that implements the above function. It has N inputs 101, 102, . . . 10N each of which is adapted to be connected to a respective optical fiber 121 122, . . . , 12N. Each fiber transmits m channels consisting of m respective carriers at different wavelengths xcex1, xcex2, . . . , xcexm modulated by pulses and by packets.
Each input 101, 102, . . . 10N is connected to a respective corresponding optical demultiplexer 131, . . . , 13N which separates the m carriers received at that input. Each carrier is then transmitted to a wavelength converter and regenerator device 181, . . . , 18N. The converted signals are recombined by an optical multiplexer 141, . . . , 14N. N signals G1, G2, . . . , GN are obtained at the output of these multiplexers. The N signals are then broadcast in m.N directions by respective couplers 151, . . . , 15N. They are then routed to mxc3x97N respective outputs 201, 202, . . . , 20mN of the switch 10 by mxc3x97N respective selector units 221, 222, . . . , 22mN. Each selector unit is in two parts, namely, for the unit 221, for example, an input selector first device 241 and a wavelength selector second device 242.
For example, the input selector device 241 has N inputs each of which is connected to respective outputs 161 to 16N of the converter and regenerator devices 141, 142, . . . 14N. The input selector device 241 selects at most one of the N received signals G1, G2, . . . , GN. The selected signal is then routed to the wavelength selector device 242. From the m carriers of the signal selected, the latter device selects one carrier to be transmitted to an output 201 of the switch 10. The signals supplied at the outputs 201, 202, . . . , 20mN are then grouped into groups of m signals having different wavelengths xcex1, xcex2, . . . , xcexm by multiplexers 141, . . . , 14N so that they can be transmitted via optical fibers 131, . . . , 13N.
In other embodiments, the number of output fibers and the number of channels per output fiber can be different from N and m, respectively. On the other hand, the total number m.N of output channels is equal to the total number of input channels.
Each input selector device 241, etc. includes at least N selector units, for example optical amplifiers used as optical gates. Each wavelength selector device 242, etc. includes at least m selector units consisting of optical amplifiers used as optical gates, for example, and wavelength-selective means. The switch 10 therefore includes at least m.N.(m+N) optical amplifiers. This large number of components is not favorable to reliability, simplicity or optimum fabrication cost. Also, each signal G1, G2, . . . , GN is broadcast to an input of each of the m.N input selector units 221, 222, . . . , 22mN. A consequence of this is that the power of each signal Gi is divided by a factor m.N.
Furthermore, after the selection effected by the first device 241, the selection operated by the second device 242 further divides the power of the signal by a factor equal at most to a value from N to m. Accordingly, in total, the power of each input signal is attenuated by at least a factor N.m.max(N,m). This reduction in the power of the signals in the selector device produces a low signal/noise ratio and therefore distortion, which becomes problematic for signals at very high bit rates, for example bit rates higher than 10 Gbit/s.
An object of the invention is to reduce significantly the number of times each signal is divided for the same total bit rate processed in this kind of switch. This increases the signal/noise ratio and higher information bit rates can therefore be achieved.
The invention provides a switch for optical signals, the switch including a number of outputs at least equal to the number N of inputs, for routing an input signal to at least one output, each input being adapted to receive signals modulating optical carriers having m different wavelengths, characterized in that it includes:
means for grouping all of the carriers received at an input of the switch into non-contiguous subsets of carriers;
selector units for routing in blocks the signals corresponding to each subset of optical carriers; and
means for dividing each subset and then transmitting all the carriers of that subset to the same output of the switch.
The above switch processes a whole subset of the set of wavelengths, i.e. a plurality of wavelengths, simultaneously, enabling the same single component to be used for each function, such as amplification and switching, instead of one component for each wavelength, and this applies up to the destination output. Accordingly, the signals are divided less than in the conventional optical switch shown in FIG. 1.
In the prior art switch, the factor by which the power of each carrier is divided is mxc3x97N before the input selector device 241. Between the input selector device 241 and a wavelength selector second device 242, the division factor is whichever is the greater of m and N. The embodiments of the invention show that in the switch according to the invention the division factor is reduced to Nxc3x97B before the input selector stage and to B between the input selector stage and the stage for selecting the subset of wavelengths, B being the total number of subsets of carriers per optical fiber. For example, if the set of carriers comprises 16 carriers (m=16), and if each subset includes four carriers (B=4), the factor by which each signal is divided will be less than in the prior art switch shown in FIG. 1 by the following amount:             m      ·              Max        ⁡                  (                      N            ,            m                    )                            B      2        =                    16        xc3x97        4                    4        xc3x97        4              =    4  
One particular embodiment of the switch according to the invention is characterized in that it further includes:
means for grouping all the subsets of carriers into non-contiguous groups of subsets;
means for routing in blocks the information corresponding to a plurality of subsets of carriers; and
means for selecting a single subset of carriers per output of the switch.
The above switch processes a plurality of subsets of carriers at the same time, enabling the same single component to be used for each function, such as amplification and switching, instead of one component for each subset.